1. Field of the Invention
The invention relates generally to an intraocular lens and more particularly to an intraocular lens for implantation in the capsular bag or ciliary sulcus after removal of the human lens with cataract surgery.
2. Description of Related Art
Cataract surgery involves the replacement of an opacified crystalline human lens with an intraocular lens implant. A technique for removing the human lens, phacoemulsification, removes the nucleus and cortex of the opacified lens while leaving the thin transparent membrane known as the capsular bag virtually intact. Generally, a scleral or corneal incision is made and the nucleus of the cataract is emulsified preserving the capsular bag. The remaining cortex of the lens is then removed using irrigation and aspiration. Next, an intraocular lens is implanted in the capsular bag and the incision is closed. In patients where the posterior capsule has been compromised during surgery, an intraocular lens may also be positioned in the cillary silcus located in front of the anterior capsule and behind the iris portion of the eye.
The lens used for either capsular bag or cillary silcus implantation may be a hard lens, typically made from glass or plastic materials, or a foldable lens typically formed from silicone, acrylic, hydrogel or other soft materials. The size of the incision is proportional to the type of lens to be implanted. For example, if a hard lens is to be implanted in the capsular bag, usually a scleral incision is used and the incision size typically ranges from 5.5 mm to 7.0 mm. Implantation of a hard lens in the cillary silcus requires an incision size of approximately 6.0 mm. However, a hard lens is incapable of being folded or otherwise manipulated for insertion through a relatively small incision as compared with the incision size required for a foldable lens. Generally, a smaller incision is preferred because of the advantages associated therewith which include greater wound stability, decreased induced astigmatism, more rapid visual and physical rehabilitation as well as reduction of associated medical and surgical expenses.
A foldable lens, on the other hand, can be used with a scleral or corneal incision and typically only requires an incision size of within the range of approximately 2.5 to 4.0 mm depending on the type of foldable lens selected. The foldable lens is compressed or folded along its longitudinal dimension, inserted through the incision, positioned within the capsular bag, and passively unfolded in the capsular bag or cillary silcus such that the lens optic portion is centered.
In its uncompressed position, a foldable lens is typically within the range of approximately 5.5-6.5 mm wide and has a longitudinal dimension of approximately 12.0 to 13.0 mm. However, in its compressed or folded position, the foldable lens is capable of passing through the approximately 2.5 to 4.0 mm incision. Typically, there are two types of foldable lenses used for cataract implantation. First, a plate lens includes an optic portion and plate haptic portions usually formed from silicone. The plate lens is folded longitudinally, inserted through the incision and positioned within the capsular bag. An injector may be used to deliver the plate lens through the wound and into position. When a plate lens is inserted into the capsular bag, the lens centers well, however over time, the lens tends to de-center due to scaring and fibrosis of the capsular bag. Another disadvantage associated with the plate lens is that because the capsular bag may contract asymmetrically, a typical plate lens may bend or sublaxate within the bag thereby changing the refractive effect of the lens. This induces unintended hyperopia, myopia or astigmatism. Another drawback associated with the plate lens occurs after laser capsulotomy, a procedure which creates an opening in the posterior capsule after cataract surgery in patients where the capsule opacifies and causes decreased vision. Specifically, because the lens is not sufficiently rigid, the lens may fold back-up due to contraction of the capsular bag and be extruded through the posterior capsular opening and into the vitreous cavity requiring corrective surgery. Another drawback associated with a plate lens is that a continuous circular opening is required to facilitate implantation. If the opening is non-continuous or jagged, the plate lens becomes unsuitable for implantation in the capsular bag because lens delivery will result in tears of the anterior capsule portion which results in inadequate fixation of the lens. A still further disadvantage with the plate lens is evident in this type of lens cannot be used for implantation in the cillary silcus. In particular, because the longitudinal size of the cillary silcus, usually approximately 13.0 mm, is larger than the longitudinal size of the plate lens which is usually approximately 12.0 mm, decentration of the lens optic portion occurs causing vision degeneracy, and therefore, a plate lens is contra-indicated for silcus implantation.
A second type of foldable lens typically used is referred to as a three piece lens. This type of lens usually includes a lens optic portion and a pair of J-shaped or C-shaped loop haptics connected to the optic portion at separate juncture sites located along the peripheral edge of the optic. The optic portion may be made of acrylic, silicone or other foldable materials. The haptic portion of the lens typically is made of polymethylmethacrylate (PMMA) or polypropylene (Prolene).
Similar to the plate lens, the three piece lens may be folded, inserted through an incision and positioned within the capsular bag. Typically, the incision size required for insertion of a three piece lens is approximately 3.0-4.2 mm. However, a disadvantage associated with the three piece lens is that it is more awkward to handle than the plate lens which makes positioning within the capsular bag more cumbersome and may result in damage to the lens, haptics or intraocular tissue during implantation. In addition, the incision size required for insertion is approximately 3.0 to 4.2 mm whereas a plate lens may be used with an incision of approximately 2.5 to 4.0 mm. Another disadvantage associated with the three piece lens is that when implanted in the capsular bag, it may decenter because the loop haptics tend to have low tensile strength to permit folding, but not sufficiently rigid to maintain positioning after insertion. This may result in decentering because of capsular fibrosis and contraction. Likewise, the three piece lens is not the preferred choice for lens implantation in the cillary silcus because of the lens' insufficient rigidity. A further disadvantage associated with the three piece lens is that bacteria tends to migrate at the juncture or connection sites between the optic and haptic portions thereby increasing the potential for post-operative infection.